Your whole comment ignores seemingly the entirety of human history other than the (sanitized) history of Western Europe and the US.
> Sex within marriage, no divorce, marriage and sex for procreation, reliance on extended family and local community, no industrialization/urbanization, social isolation, religion as unifying principle.
All of these things have been present in the most war-torn and unstable societies (ex: much of the Middle East, some of the areas around present-day China and Russia).
> Hippy communities on the other hand tended to be built around sexual promiscuity or at least lack boundaries around sex and marriage, and subsequently suffer commensurate relationship/family breakdown.
There are cultures that existed for hundreds of years without the concept of monogamous marriage. Monogamy doesn't guarantee familial stability, nor does promiscuity. Domestic partnership (what we would call marriage) was a familial and economic relationship, not always a guarantee of sexual exclusivity.
The second-largest empire in history, the Mongol Empire, had an enormous amount of polygamy and sexual promiscuity.
The Ottoman Empire, Ethiopian Empire, and many other large (and long-lasting) empires practiced polygamy.
Even ignoring all of that, in the US, most people cheat on their significant others[1]. There's no reason to think that's a modern phenomenon, either.
1. https://today.yougov.com/topics/society/articles-reports/202...